Remember (?!....) is a lookaround, so it matches 0 characters. So ^(?!....)$ will only match an empty string! (Think about it). Try ^(?!....$) in

Remember (?!....) is a lookaround, so it matches 0 characters. So ^(?!....)$ will only match an empty string! (Think about it). Try ^(?!....$) instead (the $ anchor inside the lookaround)